The brightness of a typical color television (TV) may be expressed in following equation (1) in terms of input voltage by utilizing the physical characteristic of cathode ray tube (CRT) of color TV:brightness=k1×(VINPUT/VMAX)γ  (1)where γ=2.2, k1 is a variable representing gray level of color TV (e.g., k1=256 if gray level of color TV is 256), VINPUT is input voltage varied as gray level of color TV, and VMAX is a maximum voltage required for showing a maximum gray level of color TV. Hence, the relationship of input gray level (voltage) versus output brightness of color TV may be plotted as a curve (FIG. 1b). Conventionally, prior to sending a video signal (e.g., NTSC or HDTV), a Gamma (γ) compensation process (called compensation process hereinafter) is performed on the original video signal by utilizing above physical characteristic thereof. That is, a compensation process is performed with respect to γ in equation (1). As such, the relationship of input brightness versus output gray level (voltage) of color TV may be plotted as a curve (FIG. 1a). In one example of γ=0.45 (i.e., obtained from 1/2.2), the video signal received by color TV is converted into image for showing on screen of CRT of color TV Hence, the relationship of input brightness versus output brightness of color TV may be plotted as a straight line (FIG. 1c). As a result, a high quality image is shown on the typical color TV without distortion.
As to recently available PDPs (plasma display panels) brightness of respective discharge unit on panel thereof is controlled by discharge number.
Hence, brightness may be expressed in following equation (2) in terms of discharge number as below (i.e., a straight line):brightness=k2×discharge number  (2)where k2 is a variable representing brightness in a discharge number of a pixel of plasma display panel (e.g., k2=1 if brightness of one discharge of PDP is equal to 1 cd/m2) In view of this, the higher discharge number the brighter of PDP. This is similar to the effect that the larger input voltage the brighter of a typical color TV.
Referring to FIGS. 2a, 2b and 2c, a compensation process is performed on received video signal by PDP by substituting γ=0.45 into equation (1) by similarly utilizing the physical characteristic of typical color TV. As such, the relationship of input brightness versus output gray level (voltage) of PDP may be plotted as a curve (FIG. 2a). Further, the relationship of input gray level (voltage) versus output brightness of PDP may be plotted as a straight line (FIG. 2b). Furthermore, video signal received by PDP is converted into image for showing on screen of PDP Hence, the relationship of input brightness versus output brightness of PDP may be plotted as a curve (FIG. 2c) by similarly substituting γ=0.45 into equation (1). As a result, a distorted image with poor contrast is shown on PDP.
Typically, an anti compensation process is performed for solving above drawbacks. In detail, in one example, an anti compensation process is performed on received video signal by PDP by substituting γ=2.2 into equation (1). As such, in PDP the relationship of input gray level (voltage) versus output gray level may be plotted as a curve (FIG. 3b). In another example, an anti compensation process is performed on received video signal by PDP by substituting γ=0.45 into equation (1). Hence, in PDP the relationship of input brightness versus output gray level (voltage) may be plotted as a curve (FIG. 3a). As to image shown on PDP, the relationship of input gray level (voltage) versus output brightness of PDP may be plotted as a straight line (FIG. 3c). By combining FIGS. 3a, 3b and 3c, in PDP the relationship of input brightness versus output brightness may be plotted as a straight line (FIG. 3d). In other words, a linear relationship exists between image shown on PDP and received video signal. As a result, a high quality image is shown on PDP without distortion.
As to current PDPs, signal input/output and processing are done by a digital technique. Moreover, in most cases gray level of PDP is expressed as a power of 2. For example, in PDP eight bits are needed for representing 256 gray levels. Typically, in performing a compensation process an analog-to-digital conversion is performed on video signal prior to substituting γ=0.45 into equation (1). Then an anti compensation process is performed by substituting γ=2.2 into equation (1) for effecting an inverse transform on video signal. Finally, an image is shown on PDP. However, the previous improvement technique has a disadvantage. That is, a non-integer number (e.g., decimal) can not be expressed by a digital signal. Hence, the decimal has to be converted into an integer. In the case of the original video signal having 256 gray levels, the number of gray level is reduced to 184 after first being processed in a analog-to-digital conversion and subsequently by substituting γ=2.2 into equation (1) for performing an anti compensation process thereafter. In another case that the original video signal having a gray level in the range of 0 to 40, the number of gray level is reduced to 5 (e.g., gray level 0, 1, 2, 3, and 4) after an inverse transform is performed by substituting γ=2.2 into equation (1) (see Table I below.)
TABLE Igray levelgray levelgray levelgray levelgray level ofafter γ =after γ =after γ =after γ =original video2.22.01.81.6signalconversionconversionconversionconversion 0–2000–10–20–421–2811–32–44–729–3323–45–67–934–3834–56–810–1239–4245–68–912–1443–4657–810–1114–16. . .. . .. . .. . .. . .47–616–10 8–1412–1914–25Total grayTotal grayTotal grayTotal grayTotal graylevel =level =level =level =level =6211152026
Hence, a problem of insufficient gray level of video signal is occurred in the range of low gray level after such anti compensation process. And in turn a false contour is occurred in the range of low gray level. Consequently, a poor contrast is occurred in the range of high gray level due to extremely low gray level (or brightness) gradient. As a result, a difference between two gray levels is undistinguishable visually.